Lithium-sulfur battery

ABSTRACT

A lithium-sulfur battery includes a cathode, an anode, a lithium-sulfur battery separator and an electrolyte. The lithium-sulfur battery separator includes a PSL and a FL. The FL is located on a surface of the PSL. The FL comprises carbon nanotube structure and a plurality of MoP 2  nanoparticles. The carbon nanotube structure defines a plurality of micropores. The plurality of MoP 2  nanoparticles is located on surface of the carbon nanotube structure and filled in micropores in the carbon nanotube structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. § 119 fromChina Patent Application No. 201710973046.5, filed on Oct. 18, 2017, inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. The application is also related to copendingapplications entitled, “LITHIUM-SULFUR BATTERY SEPARATOR”, filed on Sep.27, 2019 Ser. No. 16/143,481; “LITHIUM-SULFUR BATTERY SEPARATOR”, filedon Sep. 27, 2019 Ser. No. 16/143,502; “LITHIUM-SULFUR BATTERY”, filed onSep. 27, 2019 Ser. No. 16/143,503; “LITHIUM-SULFUR BATTERY SEPARATOR”,filed on Sep. 27, 2019 Ser. No. 16/503,504; and “LITHIUM-SULFURBATTERY”, filed on Sep. 27, 2019 Ser. No 16/143,506.

FIELD

The present disclosure relates to a lithium-sulfur battery.

BACKGROUND

In a lithium-sulfur battery, the cathode is made of sulfur and the anodeis made of elemental lithium. During electrical discharge process, theelemental lithium loses electrons to become lithium-ion, and the sulfurreacts with the lithium-ion and electrons to produce lithium sulfides. Areaction equation is: S₈+16Li⁺+16e⁻¹=8Li₂S. A lithium-sulfur battery hasadvantages of low-cost, environmental friendliness, good safety, andhigh theoretical specific capacity.

A separator is an important component in the lithium-sulfur battery. Theseparator separates the cathode and the anode to avoid an internalshort-circuit. A conventional lithium-sulfur battery separator ispolypropylene (PP), polyethylene (PE) or other non-polar film. However,polysulfides (Li₂S_(x), 4≤x≤8) formed during an electrical dischargeprocess can be easily dissolved into an electrolyte, which affects acyclic performance and a coulombic efficiency of the lithium-sulfurbattery. It is difficult for conventional lithium-sulfur batteryseparators to inhibit polysulfide diffusion. With a great loss of activesulfur, a “shuttle effect” occurs between electrodes. Thus the specificcapacity and cycling stability of the lithium-sulfur battery would belimited.

Therefore, there is room for improvement within the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present technology will now be described, by wayof example only, with reference to the attached figures, wherein:

FIG. 1 is a view of one embodiment of a lithium-sulfur batteryseparator.

FIG. 2 is a schematic view of one embodiment of a lithium-sulfur batteryusing the lithium-sulfur battery separator of FIG. 1.

FIG. 3 is a graph showing cyclic performance of the lithium-sulfurbattery using the lithium-sulfur battery separator (battery withMoP₂/CNT separator) with functional layer (FL) having different MoP₂nanoparticles surface density.

FIG. 4 is a graph comparing between the battery with MoP₂/CNT separatorand a lithium sulfur battery with pristine separator (PSL) (battery withPSL) of a comparative example showing cyclic performance at certaincurrent test result.

FIG. 5 is a graph comparing the battery with MoP₂/CNT separator and thebattery with PSL showing the cyclic performance at a current density of0.2 C.

FIG. 6 is a graph comparing the battery with MoP₂/CNT separator and thebattery with PSL showing a cyclic performance at high plateau capacityat 0.2 C.

FIG. 7 is a graph showing In-situ Raman spectroscopy of the battery withPSL at 2.08 V.

FIG. 8 is a graph showing In-situ Raman spectroscopy of the battery withMoP₂/CNT separator at 2.08 V.

FIG. 9 a schematic view of another embodiment of a lithium-sulfurbattery separator.

FIG. 10 a schematic view of yet another embodiment of a lithium-sulfurbattery separator.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean “at leastone.”

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures, and components havenot been described in detail so as not to obscure the related relevantfeature being described. Also, the description is not to be consideredas limiting the scope of the embodiments described herein. The drawingsare not necessarily to scale, and the proportions of certain parts havebeen exaggerated to illustrate details and features of the presentdisclosure better.

Several definitions that apply throughout this disclosure will now bepresented.

The term “substantially” is defined to be essentially conforming to theparticular dimension, shape, or other feature which is described, suchthat the component need not be exactly or strictly conforming to such afeature. The term “comprise,” when utilized, means “include, but notnecessarily limited to”; it specifically indicates open-ended inclusionor membership in the so-described combination, group, series, and thelike.

FIGS. 1 and 2 show one embodiment of a lithium-sulfur battery 10. Thelithium-sulfur battery 10 includes a cathode 20, an anode 30, alithium-sulfur battery separator 100, and an electrolyte (not shown).The electrolyte is located between the cathode 20 and the anode 30. Thelithium-sulfur battery separator 100 is located between the cathode 20and the anode 30 and is located in the electrolyte. The lithium-sulfurbattery separator 100 includes a pristine separator (hereinafter PSL)102 and a functional layer (hereinafter FL) 104 stacked with each other.The FL 104 is disposed opposite the cathode 20, that is, the FL 104 islocated between the cathode 20 and the PSL 102. In this embodiment, theFL 104 is sandwiched by the cathode 20 and the PSL 102, and the FL 104is located on the surface of the cathode 20. That is, the FL 104 and onesurface of the cathode 20 are in direct contact with each other.

The cathode 20 can be made of sulfur or composite material containingsulfur. The anode 30 can be metallic lithium. In one embodiment, theanode 30 is a lithium sheet. The electrolyte may be a solid electrolyteor a liquid electrolyte. A material of the electrolyte can be selectedfrom the electrolyte materials commonly, currently used inlithium-sulfur batteries. In one embodiment, the electrolyte is preparedby magnetically stirring 1 Mol bis-trifluoromethane sulfonamide lithium(LiTFSI) and 0.1 Mol lithium nitrate (LiNO₃) in a solvent with mixeddioxolane (DOL) and dimethyl ether (DME) for 2 hours. In the mixedsolvent, a volume ratio between DOL and DME is 1:1.

FIG. 2 shows the lithium sulfur battery separator 10 including a PSL 102and a FL 104. The PSL 102 is a planar structure and is a film. The PSL102 defines a first surface and a second surface disposed opposite thefirst surface. The first surface is opposite with the cathode 20 and thesecond surface is opposite with the anode 30. The FL 104 is located onthe first surface. The FL 104 is located between the cathode 20 and thefirst surface of PSL 102.

The PSL 102 can be a polyolefin microporous film, such as apolypropylene (PP) membrane, a polyethylene (PE) membrane, or amultilayer composite membrane of PP and PE. The PSL 102 defines aplurality of micropores through the first surface and the secondsurface. In this embodiment, the PSL 102 is a porous polypropylene film25 microns thick.

The FL 104 includes a carbon nanotube structure 106 and a plurality ofMoP₂ nanoparticles 108. That is, the FL 104 is a CNT/MoP₂ layer. Thecarbon nanotube structure 106 includes a plurality of carbon nanotubes,and the plurality of MoP₂ nanoparticles 108 are adsorbed on surfaces ofthe plurality of carbon nanotubes and supported by the carbon nanotubestructure 106. In some embodiments, the FL 104 only includes carbonnanotubes and MoP₂ nanoparticles 108, and does not contain othermaterials. The FL 104 has 3 microns to 5 microns thick.

The carbon nanotube structure 106 is a net-like structure having aplurality of micropores. The carbon nanotube structure 106 includes aplurality of carbon nanotubes. The plurality of carbon nanotubes areuniformly distributed in the carbon nanotube structure 106. The carbonnanotubes in the carbon nanotube structure 106 can be joined with eachother by van der Waals attractive force therebetween. The carbonnanotubes can be disorderly or orderly arranged in the carbon nanotubestructure 106. The term ‘disorderly’ describes the carbon nanotubesbeing arranged along many different directions, such that the number ofcarbon nanotubes arranged along each different direction can be almostthe same (e.g. uniformly disordered), and/or entangled with each other.The term ‘orderly’ describes the carbon nanotubes being arranged in aconsistently systematic manner, e.g., the carbon nanotubes are arrangedapproximately along a same direction and or have two or more sectionswithin each of which the carbon nanotubes are arranged approximatelyalong a same direction (different sections can have differentdirections). The carbon nanotubes in the carbon nanotube structure 106can be single-walled, double-walled, or multi-walled carbon nanotubes.The thickness of the carbon nanotube structure 106 is not limited, andcan be in a range from about 0.5 nanometers to about 1 centimeter. Inone embodiment, the carbon nanotube structure 106 is 1 micrometer toabout 10 micrometers thick. The carbon nanotube structure 106 caninclude at least one drawn carbon nanotube film, flocculated carbonnanotube film or pressed carbon nanotube film.

The drawn carbon nanotube film includes a plurality of successive andoriented carbon nanotubes joined end-to-end by van der Waals attractiveforce therebetween. The carbon nanotubes in the carbon nanotube film canbe substantially aligned in a single direction. The drawn carbonnanotube film can be formed by drawing a film from a carbon nanotubearray that is capable of having a film drawn therefrom. The plurality ofcarbon nanotubes in the drawn carbon nanotube film are arrangedsubstantially parallel to a surface of the drawn carbon nanotube film. Alarge number of the carbon nanotubes in the drawn carbon nanotube filmcan be oriented along a preferred orientation, meaning that a largenumber of the carbon nanotubes in the drawn carbon nanotube film arearranged substantially along the same direction. An end of one carbonnanotube is joined to another end of an adjacent carbon nanotubearranged substantially along the same direction, by van der Waalsattractive force. A small number of the carbon nanotubes are randomlyarranged in the drawn carbon nanotube film, and has a small if notnegligible effect on the larger number of the carbon nanotubes in thedrawn carbon nanotube film arranged substantially along the samedirection. The drawn carbon nanotube film is capable of forming afree-standing structure. The term “free-standing structure” includes,but is not limited to, a structure that does not have to be supported bya substrate. For example, a free-standing structure can sustain theweight of itself when it is hoisted by a portion thereof without anysignificant damage to its structural integrity. So, if the drawn carbonnanotube film is placed between two separate supporters, a portion ofthe drawn carbon nanotube film, not in contact with the two supporters,would be suspended between the two supporters and yet maintain filmstructural integrity. The free-standing structure of the drawn carbonnanotube film is realized by the successive carbon nanotubes joined endto end by van der Waals attractive force. Microscopically, in the drawncarbon nanotube film, the carbon nanotubes oriented substantially alongthe same direction may not be perfectly aligned in a straight line, andsome curve portions may exist. It can be understood that a contactbetween some carbon nanotubes located substantially side by side andoriented along the same direction can not be totally excluded. The drawncarbon nanotube film can be a pure structure only including the carbonnanotubes. The drawn carbon nanotube film has 0.5 nanometers to 100micrometers. The width and length of the drawn carbon nanotube film isnot limited. When the carbon nanotube structure 106 includes a pluralityof drawn carbon nanotube films, an angle between the aligned directionsof the carbon nanotubes in at least two drawn carbon nanotube films canbe in a range from about 0 degrees to about 90 degrees, for example canbe equal to about 0 degrees, 15 degrees, 45 degrees, 60 degrees, or 90degrees. In one embodiment, the carbon nanotube structure 106 includes20 layers of drawn carbon nanotube films overlapped with each other, andcarbon nanotube structure 106 is 2 micrometers thick.

The flocculated carbon nanotube film can include a plurality of long,curved, disordered carbon nanotubes entangled with each other. Thelength of the carbon nanotube film can be greater than 10 centimeters.The carbon nanotubes can be randomly arranged and curved in theflocculated carbon nanotube film. The carbon nanotubes can besubstantially uniformly distributed in the flocculated carbon nanotubefilm. The adjacent carbon nanotubes are acted upon by the van der Waalsattractive force there between, thereby forming an entangled structurewith micropores defined therein. Because the carbon nanotubes in theflocculated carbon nanotube film are entangled with each other, theflocculated carbon nanotube film has excellent durability, and can befashioned into desired shapes with a low risk to the integrity offlocculated carbon nanotube film. The flocculated carbon nanotube filmcan be a free-standing structure due to the carbon nanotubes beingentangled and adhered together by van der Waals attractive force therebetween. The flocculated carbon nanotube film is 1 micrometer to about 1millimeter thick. It is also understood that many of the embodiments ofthe carbon nanotube structure are flexible and do not require the use ofa structural support to maintain their structural integrity. Theflocculated carbon nanotube film can be a pure carbon nanotube film onlyincluding carbon nanotubes.

The pressed carbon nanotube film can be formed by pressing a carbonnanotube array to slant the carbon nanotubes in the carbon nanotubearray. The pressed carbon nanotube film can be a free-standing carbonnanotube film. The carbon nanotubes in the pressed carbon nanotube filmare arranged along a same direction, along more than one predetermineddifferent directions, or randomly arranged. The carbon nanotubes in thepressed carbon nanotube film can rest upon each other. Adjacent carbonnanotubes are attracted to each other and joined by van der Waalsattractive force. An angle between a primary alignment direction of thecarbon nanotubes and a surface of the pressed carbon nanotube film isabout 0 degrees to approximately 15 degrees. In some embodiments, theangle between a primary alignment direction of the carbon nanotubes anda surface of the pressed carbon nanotube film is between 0 degrees and15 degrees. The greater the pressure applied, the smaller the angleobtained. The pressed carbon nanotube film is about 1 micrometer toabout 1 millimeter thick. The pressed carbon nanotube film can be purecarbon nanotube film only including carbon nanotubes. The length andwidth of the pressed carbon nanotube film depend on the carbon nanotubearray that is pressed. If the length and width of the carbon nanotubearray is large, the pressed carbon nanotube film can have large lengthand width.

The MoP₂ nanoparticles 108 are not limited in shape and are nanometer insize. The plurality of MoP₂ nanoparticles 108 are attached to the carbonnanotube structural structure 106, attached on the surfaces of thecarbon nanotubes and filled in the micropores of the carbon nanotubestructure 106. Preferably, the plurality of MoP₂ nanoparticles 108 areevenly distributed on the carbon nanotube structure 106. Diameters ofthe MoP₂ nanoparticles 108 may be between 10 nm-500 nm. In oneembodiment, the plurality of MoP₂ nanoparticles 108 have an averagediameter of 100 nm.

In the lithium-sulfur battery separator 100, an areal density of theplurality of MoP₂ nanoparticles 108 (a mass of the MoP₂ nanoparticles108 in per unit area of the lithium-sulfur battery separator 100) is notlimited, and can be adjusted according to actual needs. Preferably, theareal density of the plurality of MoP₂ nanoparticles 108 are ranged from0.1 mg/cm² to 0.6 mg/cm². In one embodiment, the areal density of theplurality of MoP₂ nanoparticles 108 is about 0.3 mg/cm 2, a mass ratiobetween the plurality of MoP₂ nanoparticles 108 and the carbon nanotubestructure 106 in the lithium-sulfur battery separator 100 is 7.5:1.

Depending on the embodiment, certain of the steps of a method describedmay be removed, others may be added, and the sequence of steps may bealtered. The description and the claims drawn to a method may includesome indication in reference to certain steps. However, the indicationused is only to be viewed for identification purposes and not as asuggestion as to an order for the steps.

A method for preparing the lithium-sulfur battery separator 100 mayinclude at least the following steps: providing the PSL 102; laying thecarbon nanotube structure 106 on a surface of the PSL 102; and uniformlydepositing a suspension of the MoP₂ nanoparticle 108 on the carbonnanotube structure 106, after drying, a plurality of MoP₂ nanoparticles108 are attached on the surfaces of the carbon nanotubes in the carbonnanotube structure 106. The plurality of MoP₂ nanoparticles 108 arelocated on the surface of the carbon nanotube structure 106 or in themicropores of the carbon nanotube structure 106.

The lithium-sulfur battery 10 provided by the present invention uses aspecial separator, that is, the lithium-sulfur battery separator 100.The lithium-sulfur battery separator 100 includes a PSL 102 and a FL104. The FL 104 includes a carbon nanotube structure 106 and a pluralityof MoP₂ nanoparticles 108, and the FL 104 is disposed opposite thecathode 20 of the lithium sulfur battery 10. During the charging anddischarging process of the lithium-sulfur battery 10, a plurality ofMoP₂ nanoparticles 108 act as a fixing agent to chemically react withthe dissolved polysulfide to achieve the effect of adsorbingpolysulfide; meanwhile, the plurality MoP₂ nanoparticles 108 also act asa catalyst to improve redox reaction of the polysulfide to reduce thepolysulfide content in the electrolyte. Additionally, the carbonnanotube structure 106 serves as a supporting skeleton for supportingthe plurality of MoP₂ nanoparticles 108 to ensure the plurality of MoP₂nanoparticles 108 is uniformly distributed. The carbon nanotubestructure 106 also physically hinders polysulfide migration. Further,the carbon nanotube structure 106 is in contact with one surface of thecathode 20. Because of the good electrical conductivity of the carbonnanotubes, the carbon nanotube structure 106 can also be regarded as acurrent collector for providing the cathode 20 a rich electron passage.As such, the internal charge transfer resistance of the lithium-sulfurbattery 10 is reduced. Test results show that the cyclic performance ofthe lithium-sulfur battery 10 using the lithium-sulfur battery separator100 is significantly improved. A capacity attenuation per cycle is0.152% at 0.2 C in 100 cycles; the capacity attenuation per cycle is0.025% at 1 C in 500 cycles. The in-situ Raman spectroscopy also provethe adsorption effect of polysulfide by the FL 104 in the lithium-sulfurbattery separator 100 and the catalysis effect of the MoP2 nanoparticlesduring charge and discharge.

FIG. 3 shows the areal density of the MoP₂ nanoparticles 108 is also animportant parameter affecting the electrochemical performance of thelithium-sulfur battery 10. The initial capacitance of the cathode 20 isthe same. After 100 cycles, when the surface density of the MoP₂nanoparticles 108 in the lithium-sulfur battery separator 100 is low(0.1 mg/cm²), the capacitance of the cathode 20 is attenuated to 783.9mAh/g; when the areal density of MoP₂ nanoparticles 108 in the sulfurbattery separator is high (0.6 mg/cm²), the capacity of the cathode 20is attenuated to 657.5 mAh/g; when the areal density of MoP₂nanoparticles 108 in the lithium-sulfur battery separator 100 is at anintermediate value (0.3 mg/cm²), the capacity of the cathode 20 isattenuated to 905.4 mAh/g. The above testing data indicates that, theareal density of the MoP₂ nanoparticles 108 is not the higher the betteror worse. When the areal density is too low, the polysulfide may not becompletely adsorbed; when the areal density is too high, the adsorbedpolysulfide may be difficult to be released because of the stronginteraction. As such, the areal density of the MoP₂ nanoparticles isranged from 0.1 mg/cm² to 0.6 mg/cm².

Comparing tests between lithium-sulfur battery 10 using lithium-sulfurbattery separator 100 (battery with MoP₂/CNT separator) and aconventional lithium-sulfur battery using PSL (battery with PSL) areprovided to prove the effect of the lithium-sulfur battery separator100. In FIGS. 4-8, the curve corresponding to the MoP₂/CNT separatorrepresents the battery with MoP₂/CNT separator provided by the presentinvention, and the curve corresponding to the PSL represents the batterywith PSL provided by a comparative example. The diaphragm of thelithium-sulfur battery 10 provided by the present invention adopts thelithium-sulfur battery separator 100 including the PSL 102 and the FL104; the conventional lithium-sulfur battery provided adopts aconventional lithium-sulfur battery separator only including the PSL102. The conventional lithium-sulfur battery separator does not containa FL. Other features of the lithium-sulfur battery 10 and theconventional lithium-sulfur battery are the same.

FIG. 4 is a graph comparing between the battery with MoP₂/CNT separatorand a lithium sulfur battery with PSL (battery with PSL) of acomparative example showing cyclic performance at certain current testresult. FIG. 4 shows the battery with MoP₂/CNT separator exhibits ahigher cycle retention ratio than the battery with PSL. The initialspecific discharge capacity of the battery with MoP₂/CNT separator is1223.2 mAh/g, and after 100 cycles, the specific discharge capacity isattenuated to 905.4 mAh/g. That is, the capacity retention rate is74.02%, wherein, after the second discharge cycle, each cycle capacitydecay rate is only 0.152%. The battery with PSL provided in thecomparative example has a battery capacity retention rate of 33.2% (360mAh/g) after 100 cycles. Accordingly, the battery with MoP₂/CNTseparator has a better cyclic performance, sufficient to demonstrate theeffect of the sulfur-sulfur battery separator with the FL on the cyclicperformance of the entire battery.

FIG. 5 is a graph comparing the battery with MoP₂/CNT separator and thebattery with PSL showing the cyclic performance at a current density of0.2 C (1 C=1600 mA/g). In the voltage characteristic curves, a voltagehysteresis ΔE between a platform at the time of charging and a platformat the time of discharge is an important indicator of theelectrochemical reversibility in the battery system. It can be seen fromthe figure that, the ΔE value of the lithium-sulfur battery with PSL is0.32 V; the ΔE value of the lithium-sulfur battery with MoP₂/CNTseparator is 0.25 V. The lower the ΔE value of the lithium-sulfurbattery, the weaker the internal polarization of the battery, and thebetter the rate performance of the battery. The decrease in the ΔE valueof the lithium-sulfur battery with MoP₂/CNT separator is mainly due tothat the presence of MoP₂ nanoparticles in the lithium-sulfur batteryseparator improves the polysulfide conversion activity.

FIG. 6 is a graph comparing between the battery with MoP₂/CNT separatorand the battery with PSL showing a cyclic performance at high platformcapacity at 0.2 C. In the lithium-sulfur battery, the high platformcapacity corresponds to a process of conversion of elemental sulfur S toLi₂S₄. The higher the high platform capacity, the better the inhibitioneffect of the lithium-sulfur battery separator on the shuttle effect ofpolysulfide. FIG. 6 shows that, in the lithium-sulfur battery withMoP₂/CNT separator, the lithium sulfur battery separator has a betterinhibitory effect on the shuttle effect of polysulfide.

FIG. 7 is a graph showing In-situ Raman spectroscopy of the battery withPSL at 2.08 V. FIG. 8 is a graph showing In-situ Raman spectroscopy ofthe battery with MoP₂/CNT separator at 2.08 V. For the lithium-sulfurbattery with PSL. FIG. 7 shows peaks of Li₂S₆ and Li₂S₂ can be observedon both sides of the cathode and the anode, indicating that Li₂S₆ andLi₂S₂ substances on both sides of the cathode and anode. As such, thePSL does not inhibit the shuttle effect of polysulfide. For thelithium-sulfur battery with MoP₂/CNT separator, the peaks of Li₂S₆ andLi₂S₂ are observed only on the cathode side, and no polysulfide peakappears on the anode side, which indicates that the MoP₂/CNT separatorthe lithium-sulfur battery has a good inhibitory effect on the shuttleeffect of polysulfide.

Another embodiment of the present invention provides a lithium sulfurbattery comprising a cathode, an anode, a lithium-sulfur batteryseparator 200 and an electrolyte.

FIG. 9 shows the lithium-sulfur battery separator 200 provided in thesecond embodiment includes a PSL 202 and a FL 204. The PSL 202 is afilm. The PSL 202 has a first surface and a second surface opposite withthe first surface. The first surface is located opposite with thecathode, and the second surface is located opposite with the anode. TheFL 204 is disposed on the first surface. The FL 204 is located betweenthe cathode and the first surface of PSL 202.

The FL 204 includes a plurality of MoP₂ nanoparticles 208 and aplurality of carbon nanotubes 210. The plurality of MoP₂ nanoparticles208 and the plurality of carbon nanotubes 210 are uniformly mixed witheach other. The plurality of carbon nanotubes 210 can be entangled witheach other. The MoP₂ nanoparticles 208 can also be entangled by thecarbon nanotubes 210. The plurality of MoP₂ nanoparticles 208 and theplurality of carbon nanotubes 210 can be joined by van der Waalsattractive force to form an integrity structure. In the lithium-sulfurbattery separator 200, an areal density of the plurality of MoP₂nanoparticles 108 is not limited and can be adjusted according topractical needs. In some embodiments, the plurality of MoP₂nanoparticles 208 have an areal density ranged from 0.1 mg/cm² to 0.6mg/cm².

In some embodiments, the FL 204 only includes MoP₂ nanoparticles 208 andcarbon nanotubes 210, and does not contain other materials. In otherembodiments, the FL 204 can further include conductive particles, suchas, carbon black and graphene. Alternatively, the FL 204 can furtherinclude a binder for fixing the plurality of MoP₂ nanoparticles 208 andthe plurality of carbon nanotubes 210.

A preparation method of the lithium-sulfur battery separator 200includes at least the following steps: preparing a liquid mixture ofMoP₂ nanoparticles 208 and carbon nanotubes 210, and stirring themixture uniformly; providing a PSL 102, coating the liquid mixture ofMoP₂ nanoparticles 208 and carbon nanotubes 210 on the first surface ofthe PSL 102, and to form the FL 204 after drying.

Other characteristics of the lithium-sulfur battery separator 200 is thesame as the lithium-sulfur battery separator 100 discussed above.

A lithium sulfur battery comprising a cathode, an anode, alithium-sulfur battery separator 300 and an electrolyte according to yetanother embodiment is provided.

FIG. 10 shows the lithium-sulfur battery separator 300 includes a PSL302 and a FL 304. The PSL 302 is a film having a certain thickness. ThePSL 302 has a first surface and a second surface opposite with the firstsurface. The first surface is located opposite with the cathode, and thesecond surface is located opposite with the anode. The FL 304 isdisposed on the first surface. The FL 304 is located between the cathodeand the first surface of PSL 302.

The FL 304 includes a plurality of MoP₂ nanoparticles 308 and aplurality of graphene sheets 312. The plurality of MoP₂ nanoparticles308 and the plurality of graphene sheets 312 are uniformly mixed witheach other. The plurality of graphene sheets 312 can be overlapped orlaminated to form an integral graphene layer. The plurality of MoP₂nanoparticles 308 are attached to surfaces of the plurality of graphenesheets 312. In some embodiments, the FL 304 only includes MoP₂nanoparticles 308 and graphene sheets 312, and does not contain othermaterials.

The graphene sheets 312 in the graphene layer are joined with each otherby Van der Waals attractive force. The graphene sheets 312 in thegraphene layer can be arranged side by side or overlapped with eachother. Graphene sheets have good electrical conductivity. Thickness ofgraphene sheets 312 is less than 100 nm. In an embodiment, the graphenesheets 312 have a thickness ranged from 0.5 nm to 100 nm. The graphenelayer is a single layer graphene sheet to 1 mm thick.

In the lithium-sulfur battery separator 300, an areal density of theplurality of MoP2 nanoparticles 108 is not limited, and can be adjustedaccording to actual needs. In some embodiments, the plurality of MoP₂nanoparticles 208 have an areal density ranged from 0.1 mg/cm² to 0.6mg/cm².

A method for preparing the lithium-sulfur battery separator 300 includesat least the following steps: providing a certain amount of grapheneoxide sheets, placing the graphene oxide sheets into a solvent to form amixture; ultrasonically shaking the mixture to make the graphene oxidesheets dispersed uniform to form a graphene oxide sheet dispersion;forming the graphene layer by suction filtration; transferring thegraphene layer to the surface of the PSL 302; dropping a solventincluding MoP₂ nanoparticles 108 on the graphene layer and then dryingthem.

In the lithium-sulfur battery separator 300, the plurality of MoP2nanoparticles 308 and the plurality of graphene sheets 312 are uniformlydistributed; the plurality of graphene sheets 312 are stacked to form agraphene layer to allow the electrolyte to pass smoothly, and also tohinder polysulfide migration.

Other characteristics of the lithium-sulfur battery separator 300 arethe same as the lithium-sulfur battery separator 100 discussed above.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the present disclosure. Variations maybe made to the embodiments without departing from the spirit of thepresent disclosure as claimed. Elements associated with any of the aboveembodiments are envisioned to be associated with any other embodiments.The above-described embodiments illustrate the scope of the presentdisclosure but do not restrict the scope of the present disclosure.

Depending on the embodiment, certain of the steps of a method describedmay be removed, others may be added, and the sequence of steps may bealtered. The description and the claims drawn to a method may includesome indication in reference to certain steps. However, the indicationused is only to be viewed for identification purposes and not as asuggestion as to an order for the steps.

What is claimed is:
 1. A lithium-sulfur battery comprising: a cathode,an anode, a lithium-sulfur battery separator, and an electrolyte; theelectrolyte is located between the cathode and the anode; thelithium-sulfur battery separator is located between the cathode and theanode and in the electrolyte, wherein the lithium-sulfur batteryseparator comprising: a pristine separator (PSL); and a functional layer(FL) located on a surface of the PSL, wherein the FL comprises: a carbonnanotube structure defining a plurality of micropores; and a pluralityof MoP₂ nanoparticles located on a surface of the carbon nanotubestructure and filled in micropores in the carbon nanotube structure. 2.The lithium-sulfur battery of claim 1, wherein the FL faces the cathode.3. The lithium-sulfur battery of claim 2, wherein the FL is locatedbetween the PSL and the cathode.
 4. The lithium-sulfur battery of claim3, wherein the FL is contacting with a surface of the cathode.
 5. Thelithium-sulfur battery of claim 1, wherein the carbon nanotube structurecomprises a plurality of carbon nanotubes combined with each other byvan der Waals attractive force therebetween.
 6. The lithium-sulfurbattery of claim 5, wherein the plurality of carbon nanotubes areoriented along a same direction.
 7. The lithium-sulfur battery of claim5, wherein the carbon nanotube structure comprises a plurality ofsuccessive and oriented carbon nanotubes joined end-to-end by van derWaals attractive force therebetween.
 8. The lithium-sulfur battery ofclaim 1, wherein the carbon nanotube structure comprises a pluralitylayers of carbon nanotube films overlapped with each other.
 9. Thelithium-sulfur battery of claim 8, wherein each carbon nanotube filmcomprises a plurality of carbon nanotubes oriented along a samedirection, an angle between the aligned directions of the plurality ofcarbon nanotubes in at least two carbon nanotube films is in a rangefrom 0 degrees to about 90 degrees.
 10. The lithium-sulfur battery ofclaim 1, wherein diameters of the plurality of MoP₂ nanoparticles are ina range from 10 nanometers to 500 nanometers.
 11. The lithium-sulfurbattery of claim 1, wherein an area density of the plurality of MoP₂nanoparticles in the FL is in a range from 0.1 mg/cm² to 0.6 mg/cm². 12.The lithium-sulfur battery of claim 1, wherein the FL is 2 micrometersto 5 micrometers thick.
 13. The lithium-sulfur battery of claim 1,wherein FL only comprises carbon nanotubes and MoP₂ nanoparticles. 14.The lithium-sulfur battery of claim 1, wherein the carbon nanotubestructure is a free-standing structure.
 15. The lithium-sulfur batteryof claim 1, wherein the carbon nanotube structure is a supportingskeleton for supporting the plurality of MoP₂ nanoparticles.
 16. Thelithium-sulfur battery of claim 1, wherein the cathode is sulfur orcomposite material containing sulfur.
 17. The lithium-sulfur battery ofclaim 1, wherein the electrolyte is a liquid electrolyte.